1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a twisted nematic type liquid crystal display device.
2. Description of the Related Art
As liquid crystal display devices, a simple matrix type and an active matrix type have been proposed. In the single matrix type, two stripe-like electrode groups are disposed oppositely in parallel with each other with a liquid crystal being disposed therebetween, and are extended to cross each other. A plurality of crossing portions of the two electrode groups, each of which is crossed through the liquid crystal, constitute a plurality of pixels. In the active matrix type, a plurality of matrix-like pixel electrodes, in which thin film transistors are respectively connected to crossing portions thereof, and opposite electrodes, opposed to the pixel electrodes through a liquid crystal, constitute a plurality of pixels at their respective opposing portions.
Recently, active matrix type liquid crystal display devices having thin film transistors (hereinafter referred to as "TFT") have been employed for displays of word processors or personal computers. The liquid crystal display device (hereinafter referred to as "LCD") has a twisted nematic liquid crystal cell having TFTs at respective pixel electrodes, a polarizer disposed at a light incident side of the liquid crystal cell to parallel a transmission axis of the polarizer with a rubbing direction of a substrate at the light incident side of the liquid crystal cell, and an analyzer disposed at a light exit side of the liquid crystal cell to substantially perpendicularly cross a transmission axis of the analyzer over the transmission axis of the polarizer.
Since this conventional TFT-LCD can be statically driven, its contrast is high, and its viewing angle is relatively wide.
The viewing angle characteristic of the conventional TFT-LCD is shown in FIG. 1, its voltage-luminance characteristic is shown in FIG. 2, and change in displayed chromaticity is shown in FIG. 3. FIG. 1 shows equal contrast curves. Five concentric circles sequentially illustrate directions inclined at angles of 10.degree., 20.degree., 30.degree., 40.degree. and 50.degree. with respect to the normal of a display of the TFT-LCD from the center toward the outside. The equal contrast curves show contrasts observed at a rotating angle position, where a clockwise direction (counterclockwise direction in FIG. 1) is positive as seen from the light incident side of the liquid crystal cell with the rubbing direction A of the substrate at the incident light side of the liquid crystal cell used as a reference (0.degree.). A broken curve illustrates a region where the value of the contrast is 100, a one-dot chain line shows a region where the value of the contrast is 50, a solid line illustrate a region where the value of the contrast is 10, and a two-dot chain line shows a region where the value of the contrast is less than 1, i.e., the bright and dark in the display are inverted. The contrast is measured by using a CIE standard light source "C", the value of the contrast is defined by the ratio of Y value of a transmission light, and a front surface contrast is defined as about 100.
As shown by the equal contrast curves, in the conventional TFT-LCD, a direction (viewing angle direction) for obtaining a high contrast is disposed in a direction V of 315.degree. from a rubbing direction of an aligning film formed on the substrate of the light incident side. Therefore, in the TFT-LCD, the direction V is brought into coincidence with the vertical direction of the liquid crystal cell. In the TFT-LCD, the region surrounded by the one-dot chain line and having 50 or more of contrast is relatively wide, and the region surrounded by the solid line and having 10 or more of contrast is also relatively wide. Thus, the TFT-LCD has a better viewing angle characteristic than the conventional simple matrix type liquid crystal device.
However, as shown in FIG. 1, the above-described conventional liquid crystal display device has relatively wide viewing angle in the lateral directions of the liquid crystal display device, but has a disadvantage in that the viewing angle in the vertical direction is narrow. The conventional liquid crystal display device has a relatively large range of a region having less than 1 of the contrast surrounded by the two-dot chain line in a direction in which the liquid crystal display device is observed (from above) as shown in FIG. 1. This region which has 1 or less of the contrast is a region where the brightness and darkness of an image to be displayed are inverted (hereinafter referred to as "an inverted region" and hatched in the attached drawings). In the inverted region, when the liquid crystal display device is observed from a direction inclined at 40.degree. to 50.degree. upward from the normal of the liquid crystal display device, the image is observed like a negative image in which light and shade of the image are inverted, thereby remarkably reducing the display quality of the liquid crystal display device.
Accordingly, it is an object of the present invention to provide a liquid crystal display device in which its viewing angle is improved.